Multistage hydraulic pressure boosters for use in vehicular braking systems are relatively well known in the prior art. Such pressure boosters, or brake pressure intensifying apparatuses, employ a system of concentric pistons to obtain a gradual boost in wheel cylinder pressure, providing a brake pedal feel comparable to that provided by power assisted vehicular braking systems. Such prior art pressure intensifying apparatuses are illustrated in the U.S. Pat. No. 3,010,282 to Jansson, and the U.S. Pat. Nos. 3,425,222, 4,976,190, 5,048,397, and 5,480,222 to Cooney. These references disclose a self-contained apparatus installed along a hydraulic fluid line extending between a master cylinder and wheel cylinders of a vehicle. A fluid cylinder includes a cylindrical chamber and a piston assembly slidably disposed in the chamber. The piston assembly includes a first piston and an inner piston concentrically disposed in the first piston for telescopic sliding movement within the first piston. The inner piston defines a flow passage for communicating the fluid through the inner piston. A spring-loaded check valve is disposed in the flow passage of the inner piston. A compression spring biases the piston assembly toward an inlet end of the cylinder.
The brake pressure intensifying apparatus is actuated by fluid pressure delivered from the master cylinder. When an operator of the vehicle applies force to the brake pedal, fluid pressure from the master cylinder is transmitted to the inlet end of the brake pressure intensifying apparatus, moving the fluid straight through both the flow passage of the inner piston and the check valve to the wheel cylinders without providing any pressure intensification. When a pre-determined level of pressure is exerted from the master cylinder, the first piston and the inner piston move together or “lift off” against the biasing pressure of the spring and allow the check valve to close. This movement gradually increases the fluid pressure due to the fact that the inlet ends of each of the first piston and the inner piston are larger in diameter than at their respective outlet ends. As pressure increases in the master cylinder, the two pistons continue to move together until the first piston is stopped by an annular ledge extending inwardly into the chamber from the cylinder. The inner piston, however, is not so restrained and continues to move in the cylinder.
One of the deficiencies of the prior art intensifying apparatuses is that prior to lift off of the piston assembly, the biasing pressure of the compression spring on the piston assembly forces the first piston into contact with a first end wall that seals the chamber proximal to the inlet end of the fluid cylinder, thus partially blocking the flow passage and impeding the flow of fluid into the flow passage of the inner piston. As a result, fluid pressure through the intensifying apparatuses may be erratic and unpredictable during an initial braking period, i.e., prior to lift off, as the fluid seeps into the flow passage in spurts or at a variable rate, depending on minor piston movement. The erratic and unpredictable fluid pressures may affect braking feel and performance.
Another deficiency of the prior art braking systems is that the intensifying apparatuses generally increase the fluid pressure for the entire braking system, i.e., fluid pressure to each of the wheels is increased. As a result, reducer valves are ordinarily required in the fluid line going to the rear wheels of the vehicle for reducing braking pressure to the rear wheels and preventing the rear wheels from locking during a braking situation. The reducer valves add additional cost to the braking system.
Thus, there is an opportunity to provide a hydraulic brake pressure intensifying apparatus and a brake system including the intensifying apparatus that promotes smooth and controlled flow characteristics of fluid through the intensifying apparatus, that renders the fluid pressure from the inlet end of the fluid cylinder through the intensifying apparatus more consistent, and that provides an improved braking feel and performance, particularly during an initial braking period.